The present invention relates to an improved fan resistor for use in vehicles where the fan is part of the equipment for heating or air conditioning the cabin of a vehicle and the resistor is in series with the fan for adjustably setting fan speed.
The fans that circulate cabin air around the heating and cooling devices are driven by DC current from the battery and fan speed is adjusted by the provision of an adjustable resistor in series with the fan. When the fan is operating at a very low rate, most of the power is being dissipated in the resistor and the heat generated in the resistor must be dissipated to prevent failure of the resistor. Existing fan resistors are formed on a nonconductive substrate such as ceramic and applied to one surface of the nonconductive substrate is a serpentine pattern of an electrically resistive material forming the electrically resistive path of the resistor. The substrate with the serpentine pattern thereon is encapsulated in a heat conductive enclosure for transferring away the heat generated therein. It is common for the enclosure to be in the form of a pair of parallel aluminum plates with the substrate having the pattern of resistive material sandwiched between the plates. The package, including the plates, substrate and resistive pattern, is positioned near the fan such that air circulated by the fan cools the heat conductive plates and dissipates heat generated in the resistive pattern.
The serpentine pattern of electrically resistive material is presently applied to the surface of a substrate using either a printing process in which a die having an etching therein shaped in the contour of the desired serpentine pattern is filled with an electrically resistive ink. A print pad is applied to the die thereby transferring the pattern to the print pad after which the print pad is applied to the substrate to print the pattern thereon. Alternately, the serpentine pattern can be formed by first painting a central portion of the substrate with an electrically resistive paint and then using a laser to burn off undesired portions of the paint leaving only the serpentine pattern.
Regardless of whether the serpentine pattern is formed using an ink and a printing process or formed using a paint and a laser, the process of making such a fan resistor is expensive. The chemicals, ink or paint, are expensive because they require rare and expensive noble metals in their manufacture. Also, the printing or laser burning process is time consuming.